<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
</head>
<body>
    <button>one</button>
    <button>two</button>
    <button>three</button>
    <button>four</button>
    <button>five</button>

    <script>
        const btns = document.querySelectorAll('button');
        for(var i = 0; i < btns.length; i++){

            btns[i].onclick = (function(index){
                return function(){
                    console.log(index);
                }
            })(i);

/* 
            i

            {
                console.log(i);
            }

            {
                console.log(i);
            }

            {
                console.log(i);
            }



*/

            /* 
                {   i  0
                    {
                        console.log(i);
                    }
                }

                {   i    1
                    {
                        console.log(i);
                    }
                }
            
            */
        }
    </script>
</body>
</html>